1. Field of the Invention
This invention relates to an electrophotographic apparatus such as a copying apparatus or a laser beam printer.
2. Related Background Art
There is a well known image forming apparatus of a type in which a toner image electrostatically formed on the surface of an image bearing body such as a photo-sensitive drum is electrostatically transferred to a transfer material (e.g. paper) brought into close contact therewith and which uses an electrically conductive transferring roller or a corona charger as a transferring member.
In this image forming apparatus, the transferring member is urged against or brought into close vicinity to the image bearing body to thereby form a transferring portion therebetween, and the transfer material is passed through this transferring portion and a transfer bias of a polarity opposite to the polarity of the toner image on the image bearing body is applied to the above-mentioned transferring member to thereby transfer the toner image on the image bearing body onto the transfer material.
Recently, image forming apparatuses, particularly laser beam printers and digital copying apparatuses have been advanced in high resolution and an electrostatic latent image formed on the image bearing body is of a very small size. In an electrophotographic apparatus advanced in high resolution (particularly in which the electrostatic image is 600 dpi or greater), to form such an electrostatic latent image, there has been adopted in the image bearing body the technique of thinning the thickness of a photosensitive layer having a charge creating layer and a charge transporting layer from conventional 25 to 30 xcexcm to 10 to 20 xcexcm.
FIG. 2B of the accompanying drawings shows the potential distribution on the image bearing body when the thickness of the photosensitive layer is thinned to 15 xcexcm. What is shown in FIG. 2A of the accompanying drawings is a conventional one in which the thickness of the photosensitive layer is 30 xcexcm. Comparing the two with each other, in the photosensitive layer of FIG. 2B of which the thickness is 15 xcexcm, the potential in a light portion MD to which light was applied and dark portions ML to which light was not applied, is flat and the electrostatic latent image is sharp. It is also seen that in the edge portions E of the electrostatic latent image, the potential does not gently attenuate but sharply attenuates.
FIG. 4 of the accompanying drawings shows the relation between the thickness of the charge transporting layer and the image property (the reproducibility of the electrostatic latent image). It is seen from FIG. 4 that the image property becomes good from a point at which the thickness of the layer is 15 xcexcm or less.
The potential distribution as shown in FIG. 2B, i.e., an electrostatic latent image which is sharp and of which the edge portions E are upright (hereinafter simply referred to as the xe2x80x9csharp electrostatic latent imagexe2x80x9d), is very effective for forming a clear image when a toner is made to adhere to the electrostatic latent image on the image bearing body to thereby develop (visualize) it as a toner image.
In the case of the sharp electrostatic latent image, however, there has been the problem that during transfer, the toner on the image bearing body is liable to be scattered and transferred to a transfer material.
That is, when a toner image is to be transferred from a conventional image bearing body in which the thickness of the photosensitive layer is great, the electric field in the edge portions of the electrostatic latent image attenuates gently and therefore, between a toner-adhering portion and a toner-non-adhering portion, it is difficult for the transfer electric field by transfer bias to be disturbed, whereas in an image bearing body wherein the thickness of the photosensitive layer is made small, a change in the potential of the edge portions of the electrostatic latent image is sudden and therefore, in the transfer electric field, the electric field toward the transfer material is liable to be disturbed. From this disturbance of the transfer electric field, there arises the problem that the toner during transfer is liable to scatter from the toner-adhering portion to the toner-non-adhering portion. This is because as shown by a graph indicated by a dotted line in FIG. 5B of the accompanying drawings, there is a great peak in the course of the potential distribution in the transfer direction between the photosensitive body and the transfer paper in the edge portion of the latent image.
Also, a toner manufactured by the polymerizing method has recently been used. This toner is substantially spherical in its shape and has the characteristic that the transfer efficiency thereof is high. Such a characteristic is very effective for eliminating a cleaner in the image forming apparatus. That is, the transfer efficiency is high and the toner remaining on the surface of the image bearing body after transfer (untransferred toner) is small in quantity and therefore, for example, the collection of the toner by a developing device or the like is possible without providing a cleaning device for exclusive use.
However, this toner has such an excellent characteristic, but when this toner is used for the sharp electrostatic latent image as previously described, there is the problem that depending on the use environment such as temperature and humidity or the kind (e.g. thickness and quality) of the transfer material used, the scattering of the toner when the toner image on the image bearing body is transferred to the transfer material is further increased
It is an object of the present invention to provide an electrophotographic apparatus in which the thickness of the charge transporting layer of a photosensitive body is 15 xcexcm or less and which forms a sharp electrostatic image and which is suited for an image of high resolution of which the resolution of the electrostatic image is 600 dpi or greater.
It is another object of the present invention to provide an electrophotographic apparatus which forms a sharp electrostatic image and also prevents the scattering of a toner during transfer.